1. Field of the Invention
This invention resides in the fields of quality control for clinical laboratory test procedures and instrumentation, and of human Factor XIII protein and its uses.
2. Description of the Prior Art
The medical community, including practicing physicians, researchers, and clinicians of all types rely on the clinical laboratory for analytical testing of biological samples as part of routine physical examinations, and in diagnosing disease and monitoring patient progress and disease conditions, as well as similar functions and services. Among the most common biological samples that are analyzed by these laboratories are serum and plasma, although other fluids such as urine and cerebrospinal fluid are often used as well. Many analyses are performed by automated instrumentation, and in some cases large numbers of samples are analyzed simultaneously. Whether the tests are performed in this manner or on an individual basis by a laboratory technician, there are numerous sources of error that can produce spurious results.
The error that arises falls within two general classesxe2x80x94(1) spurious test results due to lapses in standard operating procedures and instrument malfunctions, and (2) analytic error. Some of the most common errors of the first class are those due to inaccurate mathematical correction for specimen dilution, misinterpretation of instrument codes, and instrument sampling errors such as bubbles in sample wells or transfer tubing or other malfunctions that result in samples whose volumes are less than standard. Of the second class, the most common type of error is that caused by calibration drift. This invention addresses errors of the first class. In proficiency testing, these errors have been shown to account for 300 false test results per one million assays. A review of the causes of laboratory errors is reported by Jenny, R. W., et al., xe2x80x9cCauses of Unsatisfactory Performance in Proficiency Testing,xe2x80x9d Clin. Chem. 46(1): 89-99 (2000), who found that inaccurate dilution corrections accounted for 21% of spurious test results in proficiency testing, and missampling in a particular automated instrument such as might be caused by air bubbles or sample clotting occurred 0.016% of the time, during use of the instrument in testing for samples from the general population. A further source of error is the use of an incorrect sample type, such as urine, cerebrospinal fluid, or other bodily fluids instead of serum, or in coagulation studies the failure to differentiate between serum and plasma.
It has now been discovered that analysis of a biological sample for human Factor XIII protein is an effective way of detecting various errors of the types discussed above. The two subunits of the protein, hereinafter referred to as xe2x80x9csubunit axe2x80x9d or xe2x80x9cFXIIIa,xe2x80x9d which is recognized in the art as the activated form of the protein, and xe2x80x9csubunit bxe2x80x9d or xe2x80x9cFXIIIb,xe2x80x9d whose function is generally unknown although speculated to be that of a carrier protein, are analyzed separately in different aspects of this invention, providing different types of information useful in detecting error. The whole FXIII is a tetramer containing two of each of the subunits, and the tetramer as well as the dissociated forms of each subunit are present in human plasma. Immmunoassays of human serum, however, detect neither the tetramer nor dissociated subunit a, but instead detect only subunit b. A small amount of tetramer may be present in human serum, but if so, the amount is below the detection limit of a typical immunoassay. The terms xe2x80x9cserumxe2x80x9d and xe2x80x9cplasmaxe2x80x9d as used herein refer to human serum and plasma unless otherwise noted.
In one aspect, the present invention resides in a utilization of the fact that subunit b has a narrow physiological range in both plasma and serum and is only rarely deficient. In addition, disease states have sufficiently little effect on the concentration of this subunit. Accordingly, this aspect of the invention resides in a quantitative determination of the subunit b in a sample of serum or plasma as an indication of the amount or volume of that sample. If the amount of subunit b detected is significantly less than would be present in the sample if the sample were of the intended volume, the determination serves as an indication of a sampling volume error, i.e., a shortage relative to the intended volume of the sample.
In another aspect, this invention resides in a method for determining whether a sample is serum or plasma, or for verifying that the sample is indeed serum rather than plasma, by analyzing the sample for subunit a, whose presence serves as an indication that plasma constitutes at least part, if not all, of the sample composition.
In a third aspect, the invention resides in a method for verifying that a sample that is thought to be serum or plasma is indeed one of these two rather than another biological fluid such as cerebrospinal fluid or urine or non-human serum. This determination is achieved by analyzing the sample for the presence of subunit b, a positive result indicating that the sample is indeed serum or plasma, since subunit b is present in both serum and plasma and is not present in non-human serum or in other biological fluids.
A still further aspect of the invention resides in the analysis of a sample of serum or plasma for the quantity or concentration of subunit b in that sample to determine or verify the degree of dilution of the sample. This is achieved by comparing the quantity or concentration detected with that of a sample of the same type of fluid but whose degree of dilution is known.
These and other features and aspects of the invention will be more readily understood from the following description.